1. Field of the Invention
This invention relates to the observation optical system of a telescope, binoculars, a microscope or the like, and particularly to the magnification changing optical device thereof.
2. Description of the Prior Art
There is known a method of providing a magnification changing mechanism for an objective lens or an eyepiece or interchanging the objective lens or the eyepiece to effect a magnification change in a telescope or a microscope. The latter is much simpler in mechanism than the former, but it requires the cumbersome procedure of interchange and an optical system for interchange and is unsuitable for optical machines to the portability of which importance is attached, for example, binoculars or the like. Further, if the field stop is located at an inaccurate position, the marginal edge of the field of view is blurred and therefore, it is necessary to provide the field stop always at an optimum position even after a magnification change.